Selective hydrogenation of 3, 6-endoxy-3, 6-dihydrophthalic acid esters



a. v1 at en United States Patent O SELECTIVE HYDROGENATION OF 3,6-ENDOXY 3,6-DIHYDROPHTHALIC ACID ESTERS Helmuth Cords, Jamesburg, NJ., assignor to Olin Mamieson Chemical Corporation, New York, N.Y.,;a corporation of Virginia f.

No Drawing. Original application Dec. 14, 1955,53. No. 552,987, now Patent No. 2,866,794, dated Dec. '30, 1958. Divided and this application Mar. 19, 1958, Ser. No. 722,371

5 Claims. (Cl. 260-3475) This application is a division of my application, Serial Number 552,987, filed December 14, 1955, now Patent No. 2,866,794. This invention relates to an improved process for selectively hydrogenating chemical compounds, and more particularly to the selective hydrogenation of a 3,6-endoxy-3,6-dihydrophthalic acid ester to the corresponding 3,6-endoxy-3,4,5,6-tetrahydrophthalic acid ester.

The synthesis of pyridoxine from turan involves the utilization of the intermediate compound, the diester of furan-3,4-dicarboxylic acid (see US. patent application of Clauson-Kaas et 211., Serial No. 471,896, filed November 29, 1954, now Patent No. 2,806,852). This intermediate can be prepared in accordance with the process of Alder and Rickert [Berichte der deutschen chemischen Gesellschaft, vol. 70, p. 1354 (1937)] by reacting furan with a diester of acetylene dicarboxylic acid (e.g. the diethyl ester) to form the Diels-Alder adduct, the diester (e.g. the diethyl ester) of 3,6-endoxy-3,6-dihydrophthalic acid, which is then partially hydrogenated to form the corresponding diester of 3,6-endoxy -3,4.5,6- tetrahydrophthalic acid; the latter is then pyrolyzed to the corresponding diester of 3,4-furandicarboxylic acid. Experience has shown that the partial hydrogenation is the critical step in series, since care must be taken that only one mole of hydrogen is taken up per mole of adduct, thereby preventing the formation of the undosired totally-saturated derivative, the diester of 3,6-endoxyhexahydrophthalic acid.

An object of this invention, therefore, is the provision of a process for selectively hydrogenating a diester of 3,6-endoxy-3,6-dihydrophthalic acid to produce the corresponding diester of 3,6-endoxy-3,4,5,6-tetrahydrophthalic acid to the exclusion of the totally-saturated hexahydrophthalic acid derivative.

Another object of this invention is the provision of a process for selectively hydrogenating a diester of 3,6- endoxy-3,6'dihydrophthalic acid in high yield and good purity.

These objects are achieved by the process of this invention, which essentially comprises hydrogenating a diester of 3,6-endoxy-3,6-dihydrophthalic acid by reacting the diester with hydrogen in the presence of sponge nickel catalyst and a solvent selected from the class consisting of trichloroethylene and chlorobenzene. The resultant diester of 3,6-endoxy-3,4,5,6-tetrahydrophthalic acid can either be recovered, or be converted in situ by pyrolysis to the corresponding diester of furan 3,4-dicarboxylate. I have found that by conducting the hydrogenation reaction in a solvent selected from the class consisting of trichloroethylene and chlorobenzene, the reaction ceases (or the rate of reaction substantially decreases) after the uptake of one mole of hydrogen per mole of ester to yield the desired partially hydrogenated 3,4,5,6-tetrahydrophthalate in lieu of and to the substantial exclusion of the undesired perhydrogenated product.

The trichloroethylene and chlorobenzene may be used eitheri'sing'lyor in'combination with other halogenated hydrocarbon or with non-halogenated solvents, such as polar solvents as exemplified by butanol and acetone or non-polar solvents as exemplified by benzene. The amount of total solvent present is sufiicient to completely dissolve the 3,6-endoxy-3,6-dihydrophthalicacid ester reactant, although of course any excess thereover can be used.

, The hydrogenation can be conducted in all other respects 'in anianner well known in the art. In order to obtain maximum yields of the desired partially-hydrogenated product, at least one mole of hydrogen is used per mole of dihydrophthalate. Any excess thereover is operative, however, since the use of the trichloroethylene and chlorobenzene solvents of this invention have the unique property of inhibiting the further hydrogenation of the desired tetrahydrophthalate product to form the urn desired hexahydro (or perhydro)-phthalate.

Although any diester of 3,6-endoxy-3,6-dihydro phthalic acid can be used, the preferred esters are those of lower alkanols (e.g. the dimethyl, the diethyl, the dipropyl and the dibutyl esters).

The hydrogenation reaction can be conducted at any normal pressure and temperature including superatmospheric pressure and temperature; but for convenience, it is preferably carried out at ambient pressure and temperature.

The fol-lowing examples illustrate the invention:

Example 1 9.1 g. of dibutyl 3,6-endoxy-3,G-dihydrophthalate in ml. of trichloroethylene is hydrogenated under atmospheric pressure in the presence of 1.0 g. of sponge nickel catalyst. After the uptake of 700 ml. of hydrogen in one-half hour, there is no further consumption of hydrogen. The catalyst is filtered off, the dibutyl 3,6- endoxy-3,4,5,64etrahydrophthalate is pyrolyzed at C. and the resulting dibutyl furan-3,4-dicarboxylate distilled in vacuo (122-125" C./0.1 mm. mercury). Yield about 21.0 g. (78.2%).

Example 2 9.1 g. of dibutyl 3,6-end0xy-3,6-dihydrophthalate in 100 ml. of chlorobenzene is hydrogenated under atmospheric pressure in the presence of 800 mg. of sponge nickel catalyst. After the uptake of 800 ml. of hydrogen in three-quarters of an hour, there is no further consumption of hydrogen. Further treatment in accordance with the procedure of Example 1 yields dibutyl furan-3,4-dicarboxylate.

By way of contrast, I have found that if acetone is substituted for the trichloroethylene and the chlorobenzene in the above examples, invariably two'moles of hydrogen were taken up (at atmospheric pressure) when attempts were made to selectively hydrogenate either the dimethyl or the dibutyl ester of 3,6-endoxy-3,6-dihydrophthalic acid. These same results were observed when other solvents (specifically methanol, 10% aqueous methanol, 2 N methanolic hydrochloric acid, ethanol, acetone, ethyl acetate, acetic acid, benzene and hexane) were substituted for the halogenated hydrocarbons, or when the reaction was conducted in the absence of any solvent. It is seen, there fore, that the trichloroethylene and chlorobenzene used as solvents in the process of this invention havethe unique property, not shared by other commonly used polar and nonpolar organic solvents, of inhibiting the addition of a second mole of hydrogen in the selective hydrogenation of a 3,6-endoxy-3,6-dihydrophthalate to the desired partially-hydrogenated 3,6 endoxy 3,4,5,6 tetrahydro- Y phthalate.

The invention may be variously otherwise embodied within the scope of the appended claims. 

1. A PROCESS FOR SELECTIVELY HYDROGENATING A 3,6-ENDOXY3,-DIHYDROPHTHALATE, WHICH COMPRISES REACTING A DIESTER OF 3,6-ENDOXY-3,6-DIHYDROPHTHATIC ACID WITH HYDROGEN IN THE PRESENCE OF SPONGE NICKEL CATALYST AND A SOLVENT SELECTED FROM THE CLASS CONSISTING OF TRICHLOROETHYLENE AND CHLOROBENZENE, AND RECOVERING THE 3,6-ENDOXY-3,4,5,6TETRAHYDROPHTHALATE FORMED. 